Traditionally, vaccines have been based on live attenuated or inactivated pathogens. These strategies are inefficient, however, largely because of the antigenic variability of pathogens (e.g., viruses). Several peptide vaccines that comprise antigenic peptides or peptide fragments of pathogens have been developed. Conserved peptide fragments are less likely to exhibit antigenic variability and can overcome some of the problems associated with traditional peptides. Accordingly, subunit vaccines have been developed, which target conserved regions of pathogens. Synthetic peptide vaccines tend to be poorly immunogenic, however. The poor immunogenicity of synthetic peptide vaccines may be attributed to the fact that although these types of vaccines induce humoral antibody responses, they are less likely to induce cell-mediated responses.
Several investigators have sought to improve the antigenicity of synthetic peptide vaccines. For example, Klein et al. describe the engineering of chimeric proteins that comprise an immunogenic region of a protein from a first antigen linked to an immunogenic region from a second pathogen. (See, U.S. Pat. Nos. 6,033,668; 6,017,539; 5,998,169; and 5,968,776). Others have sought to create chimeric proteins that couple B-cell epitopes to universal T-cell epitopes in order to improve the immune response. (See, e.g., U.S. Pat. No. 5,114,713). Russell-Jones et al. (U.S. Pat. No. 5,928,644) also disclose T-cell epitopes derived from the TraT protein of Escherichia coli, which are used to produce hybrid molecules so as to generate an immune response to parasites, soluble factors (e.g., LSH) and viruses. Further, Ruslan (U.S. Patent Application Publication No. 20030232055) discloses the manufacture of vaccines based on PAMPs and immunogenic antigens. Despite these advances, the development of compositions and methods that improve the antigenicity of immunogens is manifest.